Strong interfacial Dzyaloshinskii–Moriya induced in Co due to contact with NiO

The magnetic properties of NiO/Co/Pt as a function of Co layer thickness were investigated by polar magneto-optical Kerr effect (PMOKE) (magnetometry and microscopy) and Brillouin Light Scattering (BLS) spectroscopy. PMOKE measurements revealed strong surface anisotropy (1.8 mJ/m2) favoring perpendicular magnetic anisotropy and asymmetric domain wall propagation explained by anticlockwise chirality. BLS measurements show that this chirality is induced by strong interfacial Dzyaloshinskii–Moriya interaction (+ 2.0 pJ/m). This is one of the highest values reported so far for Co layers surrounded by different layers. The observed chirality is opposite to what has been found in Co/oxide interfaces. These results and data published earlier, indicate that the strength of interfacial Dzyaloshinskii–Moriya interaction increases with the amount of stoichiometric NiO. Therefore, this work shows that NiO is the source of the interfacial Dzyaloshinskii–Moriya interaction.


Results
Magnetooptical studies. The magnetic ordering of the Co layer is determined from large field of view PMOKE microscope images (differential with respect to saturated state) registered in remanence after out-ofplane magnetic field saturation (Fig. 1a). The bright areas in this PMOKE image roughly correspond to the out-of-plane magnetization state; the dark areas, to the in-plane magnetization state of the thick Co layer, and to the non-ferromagnetic state of the thin Co layer. These data show that the PMA is observed in a wide d Co range if Co is surrounded by NiO and Pt layers. To analyze this effect, we measured magnetization reversal along the Co wedge (as a function of d Co ) using PMOKE magnetometry. Figure 1a presents three exemplary hysteresis loops for d Co = 0.5, 1.5, 2 nm. From these measurements, we were able to determine d Co dependence of basic magnetic and magnetooptical parameters like coercive field H C ; ellipticity in remanence (ɛ REM ); and, at the maximum magnetic field (ɛ MAX ), saturation field (H SAT ) and exchange bias field (H EB ). These data confirm that the PMA is preserved in a wide d Co range showing the rectangular shape of the hysteresis loop (ɛ REM /ɛ MAX = 1) up to the thickness for spin reorientation transition (SRT), d SRT ≅ 1.5 nm (Fig. 1b). Above d SRT , the out-of-plane easy axis direction rotates to easy-plane. Since the deposition process was performed in an external out-of-plane magnetic field, the H EB is detected with maximal value for d Co ~ 0.5 nm and decreases for smaller Co thicknesses (not shown here). The presence of the EB coupling is also manifested by the large value of H C , which is much higher than reported for non-oxide HM/Co/HM systems 14 (Fig. 1c). The reduction of H C starts at lower d Co than Figure 1. Results of PMOKE measurements. (a) Differential PMOKE microscope images registered in remanence after saturation with an out-of-plane magnetic field, where the bright area corresponds to the part of Co layer with PMA. This panel additionally shows the exemplary PMOKE hysteresis loops measured for d Co = 0.5, 1.5, 2 nm (green dots 1 to 3, respectively). On that loops the definitions of characteristic parameters: remanence (ɛ REM ), ellipticity in maximal magnetic field (ɛ MAX ) and saturation field (H SAT ) and exchange bias field (H EB ) are given graphically; (b,c) d Co dependencies of normalized Kerr signal ɛ REM /ɛ MAX and, coercivity (H C ), respectively. The green dashed vertical line indicates the Co thickness for spin reorientation transition (see Fig. 2d).  S1).
For the case of in-plane magnetization, the effective magnetic anisotropy field, H 1EEF , was determined as the saturation field H SAT obtained from PMOKE magnetization reversal (see PMOKE loop 3 in Fig. 1a). The magnetic anisotropy constant K 1EFF was calculated according to the relation K 1EFF = 1/2μ 0 H 1EFF M S , where M S is the saturation magnetization for bulk cobalt μ 0 M s = 1.45 T. In the case of out-of-plane magnetization and above d SRT , where a weak in-plane magnetic anisotropy appears (d Co ≤ 2 nm), H 1EFF was determined by fitting the Stoner-Wohlfarth model 15 to the PMOKE signal measured as a function of the in-plane magnetic field under a constant out-of-plane magnetic field to ensure single domain state (see Fig. 2a-c). Figure 2d shows the d Co dependence of K 1EFF d Co . We found that for thin Co layers (d Co ~ 0.6 nm) the K 1EFF is much stronger than for other systems (e.g. Pt/Co/Pt, Pt/Co/Ir, Ir/Co/Pt) 14,16 . This strong PMA is associated with the presence of NiO and Pt as buffer and cover layers, respectively. The Pt layer supports the PMA owing to large spin-orbit coupling 17 ; at the same time, hybridization between Co and O atoms at the NiO/Co interface can also be a source of PMA 18 . However, it was recently demonstrated that the AFM layer in the case of Co/NiO bilayer gives an additional contribution to magnetic anisotropy 19 , which vanishes above Néel temperature of CoO/NiO layers 20 . Therefore, this coupling has a sufficiently strong surface contribution, which leads to the Co layer being magnetized perpendicularly to the sample plane in wider d Co ranges than in other polycrystalline HM/Co/HM systems 19 . To determine the surface (2K S ) and volume (K V ) anisotropy contributions we used a linear fit to the K 1EFF d Co (d Co ) dependence (Fig. 2d). From this fitting, we obtained 2K S = 1.82 ± 0.06 mJ/m 2 and K V = − 1.23 ± 0.04 MJ/m 3 . Note that K V includes the shape anisotropy contribution too. This high 2K S value confirms that the surface contribution is a main source of PMA in NiO/Co/Pt. It should be emphasized that this value is also much higher than for similar NiO/Co/Au trilayers systems 11 , Au/Co/NiO 19 , Pt/Co/AlO x 21 , Ir/Co/AlO x 22 . Recent results show that NiO in contact with the Co layer might be the source of iDMI 6 ; therefore, we measured the domain propagation under a combination of in-plane H x and out-of-plane H z field pulses. iDMI has been confirmed as an origin of asymmetric domain wall propagation 23 . This effect occurs because the external in-plane magnetic field induces asymmetrical changes of the energy of Néel domain walls (N-DWs) 24 . Therefore, with the right combination of H z and H x , domain walls with core magnetizations pointing along and against H x move at different speeds causing asymmetric growth of bubble domains. The studies were performed for d Co = 0.9 nm where sufficiently large bubble-like domains appear. Because of high coercivity and magnetic anisotropy, it was necessary to use a strong H z pulse for this experiment. In this experiment, we found such asymmetric domain growth in our samples (Fig. 3) confirming the presence of a strong iDMI. By analyzing the asymmetry of domain wall propagation for different directions of H x and H z fields, we determined the chirality of NiO/Co/Pt. Our data reveals that spin configuration in the N-DW has ACW chirality, which is related to a positive iDMI constant (D S > 0). Note that D S sign agrees with data presented in Ref. 13 , which uses the reverse order of surrounding layers, namely Pt(bottom)/Co/Ni x O y (top), and found negative values of iDMI. For this system, it has also been shown that the magnitude of |D S | increases with larger concentrations of stoichiometric NiO in the nickel oxide layer. However, use of the sputtering technique for nickel oxide in Ar + O 2 atmosphere presented in Ref. 13 did not allow   www.nature.com/scientificreports/ directions of the applied in-plane magnetic field. By comparing curves for opposite field orientation, the difference Δf between frequency of Stokes (f S ) and anti-Stokes (f aS ) peaks can be obtained (Δf = f S -f aS )(see Fig. 4b).
These measurements were performed for different k values by changing the incident angle ϴ to determine the effective iDMI constant (D eff ≡D S /d Co ) from Δf(k) relation 25-27 : where γ is the gyromagnetic ratio, M s magnetization saturation of magnetic layer (for Co we used μ 0 M S = 1.45 T, γ = 170 GHz/T). Figure 4c shows examples of Δf(k) dependencies for different Co thicknesses (d Co = 1.35 and 1.82 nm) measured for positive and negative directions of the applied in-plane magnetic fields. From the analysis of the results shown in Fig. 4c we found this typical behavior for samples with iDMI: (i) the sign of the slope in Δf(k) dependence reverses with the field direction; (ii) this slope (corresponds to the increase of D eff ) increases as d Co decreases. Since the BLS measurements were performed along the Co wedge, the value of D eff was determined for a wide d Co range. Figure 5 presents the dependence of the product D eff ⋅d Co as a function of d Co . The iDMI constant (D s ) can be determined since this value is independent of d Co in the measured range. The value obtained, D S = + 2.0 pJ/m, is higher than those observed in similar systems (e.g. Au/Co/NiO trilayers where D S = − 1.11 pJ/m 8,12 and Pt/ Co/Ni y O x where − 0.5 < D S < − 1.2 pJ/m depending on Ar pressure during deposition 13 ). Note that, as the order of the cobalt layer covers is reversed 13 , the sign of D S changes too; therefore, the chirality set by NiO is the same. Moreover, considering the data from Ref. 13 and our data, we infer that the magnitude of the D S increases together with the concentration of NiO phase. It should be mentioned that the relative NiO concentration in 13 was obtained as a ratio of 529.9 eV to 532.6 eV XPS peaks of O 1 s spectra, whereas in 11 100% NiO concentration was deduced from Ni 2p spectra. The composition of NiO layer deposited by PLD was also evaluated from the reference sample (50 nm of NiO) described in the Supplementary Materials (Fig. S2). This allows us to to show that there is linear dependence of D S on stoichiometric NiO phase concentration (inset in Fig. 5). Note that the deposition process of NiO was much different for both systems (Ni x O y was deposited by reactive sputtering 13 , while NiO was deposited by PLD 11 ); therefore correlation between iDMI and the amount of NiO phase might be a universal relationship. This reveals that DMI can be tailored by tuning the quality of the NiO layer with respect to defects and/or the presence of Ni 2 O 3 phase in the nickel oxide layer. Interestingly, the NiO phase is responsible for stronger iDMI favoring clockwise chirality if it is on the Co layer and anticlockwise chirality when Co is on top of the NiO layer. This might be quite surprising because it is the opposite of what was previously found for different systems (oxide/Co/Pt 21 and Pt/Co/oxide 21,[28][29][30]. Considering that the iDMI originates from an exchange interaction between two neighboring FM spins coupled via a neighboring layer, we suppose that the additional contribution to iDMI comes from the interaction between Co and the antiferromagnetic NiO phase.

Conclusion
We have investigated the changes of the magnetic properties in the NiO/Co/Pt trilayer as a function of Co layer thickness. Magnetooptical studies reveal significant value of the magnetic anisotropy occurring in the PMA region and anticlockwise spin chirality within the Néel domain walls. BLS measurements confirm the presence of iDMI with a high value of D S = + 2.0 pJ/m. This value is higher than that of similar systems where the Co layer was in contact with NiO and systems where Co was in contact with other nonmagnetic oxides. The dependence between iDMI and the relative phase concentration of NIO shows that NiO is an additional source of DMI with a chirality that is opposite to that of other nonmagnetic oxides. www.nature.com/scientificreports/

Methods
Samples with nominal structures Ti(4 nm)/Au(60 nm)/NiO(10 nm)/Co(0 ÷ 3 nm, wedge)/Pt(5 nm) were deposited on thermally oxidized silicon substrate using magnetron sputtering (Ti, Au, Pt and Co layers) or PLD (NiO layer) in a UHV multichamber system with base pressure below 2 × 10 −8 mbar. The nominal thicknesses of the Ti, Au, Co, Pt, and NiO were calculated using deposition time and the deposition rate determined by X-ray reflectivity and/or profilometer measurements from calibration samples. The Co thickness was linearly varied along the sample with a slope of 0.15 nm/mm. This was realized using a moving shutter (slightly above the substrate) with constant velocity during deposition. For correct stoichiometry, the NiO layer was deposited in an oxygen-rich atmosphere (p O = 1.5 × 10 −5 mbar) in a separate UHV chamber 11 . After NiO deposition, the sample was transferred to magnetron sputtering chamber without breaking UHV conditions (during transfer, p ≤ 5 × 10 −8 mbar). This reduces oxidation of the Co layer, as discussed in our earlier work 8 . During deposition, the sample was kept in an external out-of-plane magnetic field μ 0 H dep = − 0.19 T to increase EB coupling between Co layer and NiO layers. As the NiO layer is covered by the Co and Pt layers the evaluation of the NiO stoichiometry is not possible from direct XPS measurements. To verify the composition of the NiO layer the XPS measurements were performed on the 50 nm thick NiO deposited onto Si substrate in the same condition as for the full layer stack.
The results are presented in the Supplementary Materials (Fig. S2). PMOKE magnetometry measurements were performed using a laser beam with wavelength 635 nm focused on 0.3 mm spot at nearly normal light incidence.
A PMOKE microscope equipped with a CCD camera and an appropriate digital image processing software (in PMOKE configuration the measured signal intensity is proportional to the out-of-plane magnetization component) were used to: (i) record large field of view image of remanent magnetization distributions after out-ofplane saturation; (ii) determine local magnetization loops in the whole sample area with 10 µm lateral resolution. Asymmetric domain propagation was measured using PMOKE microscope with a pulsed magnetic field applied with simultaneous out-of-plane and in-plane components. All recorded PMOKE images were presented as differential images that were obtained after, pixel-by-pixel, gray scale intensity subtraction of the reference image obtained in initial state (with small bubble domain).
BLS experiments were performed in the backscattering configuration, illuminating the sample with p-polarized laser beam with a wavelength λ = 532 nm and beam spot size about 20 μm at variable incident angle Θ. In this geometry, the value of excited (absorbed) in-plane magnon wave vector k is equal to 4π/λsinΘ. The laser power was less than 70 mW to avoid sample overheating. The scattered light was collected by a focusing lens and its spectrum was analyzed by a Sandercock-type multi-pass tandem Fabry-Perot TFP-2 HC interferometer. The external magnetic field H y was applied in the sample plane and normal to the scattering plane of the laser light. More detailed description of used BLS measurements is given in 31 .
All measurements were performed at RT.

Data availability
All results presented in this paper are available from the corresponding author on reasonable request.